The invention pertains generally to a pulsed power supply for capacitive loads, such as an electrostatic precipitator, and is more particularly directed to such power supplies with means for providing efficient energy transfer from a single supply which produces a peak pulse voltage and a residual interpulse voltage.
Conventionally electrostatic precipitators are used for scrubbing effluents or other fluids which contain particulate debris. The distribution of particles entrained in a carrier fluid, usually a waste gas or polluted air etc., can be reduced significantly by charging the particulate matter with ionized charges to where they obtain a specific polarity. The charged particulates are then moved under the influence of a high voltage electrostatic field to where they are precipitated on a collector plate of the opposite polarity. In general, many precipitators use a negatively charged electrode (cathode) at a high voltage to generate an ionizing cloud of electrons and a positively charged electrode (anode) to collect the particulates. The cathode generally uses the corona discharge method to form an ionizing cloud of electrons which charge the particulates with a negative polarity. The particulates are then moved to the positive collecting plate by the forces generated by the collection field formed between the anode and cathode.
The method by which power is supplied to an electrostatic precipitator is critical to the efficient operation of the precipitator for collecting the particles, and for the minimum use of power by the supply. In the past, dual pulsed power supplies have been shown to be advantageous where a high voltage DC collection field is impressed across the precipitator plates by one power supply, and thereafter a high voltage pulse generated by another power supply superimposed thereon. The superimposed pulses enhance the creation of ions with which to charge particulates and the high voltage collection field maintained between pulses, the interpulse voltage, provides an efficient means to produce high collection forces on the particulates.
The prior art dual supply systems, while providing the advantages of a pulsed supply to increase the efficiency of the collection process are, however, disadvantageous for at least two reasons. Initially, the high voltage DC supply, which generates the collection field, is usually formed of a transformer-rectifier set which has a low output impedance. When a voltage in excess of the breakdown voltage is impressed across the precipitator plates, an arc may form thereby drawing excessive amounts of power from the collection field supply. In addition, in these dual supplys excessive numbers of high voltage components are needed for the transformation of the line voltage to the high tension precipitator voltages, which can be several tens of KV in magnitude. It would, therefore, be advantageous and more economical to provide a collection field with a superimposed pulse voltage from a single composite supply which, in addition, employs fewer components.
It is known in such pulsed precipitators that the efficiency of the precipitator itself increases with increasing pulse voltage. This is accounted for because the corona discharge current generates an increasing number of charges as the voltage increases. However, with increased voltage there is also an increased probability of a spark forming. Modern pulsed generators attempt to solve this problem by providing a narrow pulse width which, although in excess of the breakdown or the sparking voltage of the precipitator, is narrow enough in time duration to prevent a spark from forming. Therefore, an efficient precipitator power supply should supply the voltage pulses at a peak value in excess of the DC breakdown voltage to generate increased ionization but short enough in duration to prevent sparking.
To provide a truly efficient power supply for electrostatic precipitators, the nature of the precipitator load presented to the power supply should also be taken into account. The precipitator can be viewed as a capacitive load having a nonlinear resistance in parallel therewith. The capacitance of the precipitator stores the voltage impressed thereon as a collected charge, and dissipates a portion of the charge while generating the corona discharge current during pulse periods. This discharge current of the corona electrode is the real part of the power dissipated by the precipitator which, in addition, may dissipate any unused reactive portion of the power delivered to it as heat. Therefore, a power supply having means to return unused power from the precipitator to the charging supply would greatly enhance the efficiency of the system. If this unused energy component can be returned to the power supply before it is dissipated as heat, then the actual power required by the precipitator can be reduced.
Some power supplies return unused energy from a capacitive load such as resonant supplies. These resonant supplies are however used in other high power applications such as radio transmitting equipment. These supplies are generally disadvantageous for precipitators because they lack a means to maintan a voltage between pulses for particle collection. It is known that the collection field on a precipitator during the interpulse period should be maintained at or near the corona threshold value where ions are emitted.